1. Field of the Invention
The present invention relates to a novel ethylene copolymer and a method of producing same. The ethylene copolymer of the present invention has excellent light stability. The present invention also provides a method of preparing an ethylene copolymer having excellent weatherability in stable conditions wherein residual amounts of unreacted vinyl monomer are strongly suppressed and further the reaction system does not become fouled with the vinyl monomer.
2. Description of the Related Art
Ethylene copolymers are low cost and have excellent moldability as well as optical and physical properties such as transparency and strength and are therefore, applied to practical use in a wide field.
However, they are poor in weather resistance, so there are some restrictions to outdoor applications exposed to direct sunlight. As means of improving weather resistance, it is a general technique to mix the copolymer with a benzophenone family of ultraviolet absorber, a benzotriazole family of ultraviolet absorber, a hindered amine family of light stabilizer etc. However, since these additives will bleed or bloom out gradually, they have insufficient stabilizing effect. In particular, products of small thickness such as film or the like are very prone to bleedout.
It has, therefore, been proposed to make high molecular weight additives to decrease bleedout characteristics. In Japanese Patent Laid-open No. 137358/1982 and U.S. Pat. No. 4,104,248, for example, such proposals are disclosed, and as far as bleedout is concerned the effect caused by increasing the molecular weight by polymerization of the additives can be expected; however, a satisfactory effect is not necessarily exerted owing to possibly insufficient dispersibility thereof when the agent is added to a non-polar polymer such as polyethylene.
In European Patent Laid-open No. 496, there is disclosed a polymer comprising a homopolymer of a vinyl compound having hindered amine in its side chain and a copolymer of it with a polymerizable compound containing at least one double bond, and in European Patent Laid-open No. 63544 discloses an analogous copolymer comprising vinyl compound having hindered amine in its side chain and ethylene.
The polymer of EP-A-496 is obtained by homopolymerizing, in an organic solvent such as benzene, toluene or the like, vinyl compounds respectively which have hindered amines in the side chain, or by copolymerizing said vinyl compound with other copolymerizable monomer under the conditions of adding a free radical initiator such as .alpha.,.alpha.'-azo-isobutyronitrile or the like but applying no extra pressure thereto. In the case of the copolymerization, owing to the difference in the copolymerization reactivity ratio between the polymerizable compound having at least one double bond and the vinyl compound having hindered amine in its side chain, there often developed so-called block structures comprising at least two vinyl compounds, having hindered amines in the side chain, successively bonded; therefore, when the copolymer is used solely or used as a polyblend with other polymers such as polyolefin, there occurs a problem of inadequate photostability.
On the other hand, it is intended in EP-A-63544 to provide an analogous copolymer comprising ethylene and a compound having hindered amine in its side chain, said substance being obtained by an ester interchange reaction between an ethylene/ethyl acrylate copolymer or an ethylene/methyl acrylate copolymer and alcohol having a hindered amine group. Since the ester interchange reaction is a reversible reaction, there remains unavoidably small amounts of acrylate residue, limiting light stabilizing effect of the substance.
A copolymer is also proposed in EP-A-496 which comprises a homopolymer of a vinyl compound having hindered amine in a side chain and a copolymer of it with a polymerizable compound containing at least one double bond having good solubility or compatibility with polymers, and excellent non-extractability from polymers. Nevertheless, nothing is disclosed in the patent regarding a copolymer comprising ethylene.